1. Field of the Invention
The present invention relates to a structure of and a method for manufacturing a spark plug for an internal combustion engine.
2. Description of the Related Art
Conventionally, in a spark plug used in an internal combustion engine, in order to prevent occurrence of dielectric breakdown on the surface of the insulator, a center electrode whose tip projects from the insulator is held in an axial bore formed in the insulator such that the other end of the center electrode is glass-sealed, and, by means of packing, a portion of the surface of the insulator corresponding to the glass-sealed portion of the center electrode is isolated from the high-temperature, high-pressure working atmosphere of the spark plug.
However, the insulator which constitutes the spark plug, particularly an intermediate body portion of the insulator united with a metallic shell, is always subjected to radial pressure applied from the inside by the glass seal that seals the end of the center electrode within the axial bore formed in the insulator. The intermediate body portion of the insulator is thinner in wall thickness than the remainder of the insulator and, at the time of spark discharge, a high voltage on the order of tens of kilovolts is applied between the intermediate body portion of the insulator and an intermediate hole portion of the metallic shell. Further, a dent, protrusion or a fine defect such as a crack can be formed on the surface of the insulator, particularly on the surface of the intermediate body portion of the insulator closely facing the intermediate hole portion of the metallic shell, during sintering of the insulator or during assembly of the metallic shell and the insulator. Upon the above application of high voltage, electric field concentration is apt to occur at the above dent or protrusion or like defect present on the surface of the insulator due to a great change in dielectric constant between the surface of the insulator and an ambient air layer, potentially resulting in dielectric breakdown and thus misfire at the time of spark discharge.